The invention relates generally to plasma sputtering. In particular, the invention relates to auxiliary sources of magnetic field in magnetron sputtering.
Magnetron sputtering is a principal method of depositing metal onto a semiconductor integrated circuit during its fabrication in order to form electrically connections and other structures in the integrated circuit. A target is composed of the metal to be deposited, and ions in a plasma are attracted to the target at sufficient energy that target atoms are dislodged from the target, that is, sputtered. The sputtered atoms travel generally ballistically toward the wafer being sputter coated, and the metal atoms are deposited on the wafer in metallic form. Alternatively, the metal atoms react with another gas in the plasma, for example, nitrogen, to reactively deposit a metal compound on the wafer. Reactive sputtering is often used to form thin barrier and nucleation layers of titanium nitride or tantalum nitride on the sides of narrow holes.
DC magnetron sputtering is the most usually practiced commercial form of sputtering. The metallic target is biased to a negative DC bias in the range of about xe2x88x92400 to xe2x88x92600VDC to attract positive ions of the argon working gas toward the target to sputter the metal atoms. Usually, the sides of the sputter reactor are covered with a shield to protect the chamber walls from sputter deposition. The shield is usually electrically grounded and thus provides an anode in opposition to the target cathode to capacitively couple the DC target power into the chamber and its plasma.
A magnetron having at least a pair of opposed magnetic poles is disposed in back of the target to generate a magnetic field close to and parallel to the front face of the target. The magnetic field traps electrons, and, for charge neutrality in the plasma, additional argon ions are attracted into the region adjacent to the magnetron to form there a high-density plasma. Thereby, the sputtering rate is increased.
However, conventional sputtering presents challenges in the formation of advanced integrated circuits. As mentioned above, sputtering is fundamentally a ballistic process having an approximate isotropic sputtering pattern that is well suited for coating planar surfaces but ill suited for depositing metal into the narrow features characteristic of advanced integrated circuits. For example, advanced integrated circuits include many inter-level vias having aspect ratios of 5:1 and higher, which need to be coated and filled with metal. However, techniques have been developed for drawing the sputtered atoms deep within the narrow, deep holes to coat the bottom and sides and then to fill the hole with metal without bridging the hole and thereby forming an included void.
A general technique for sputtering into deep holes is to cause the sputtered atoms to be ionized and to additionally negatively bias the wafer to cause the positively charged sputtered metal atoms to accelerate toward the wafer. Thereby, the sputtering pattern becomes anisotropic and directed toward the bottom of the holes. A negative self-bias naturally develops on an electrically floating pedestal. However, for more control, a voltage may be impressed on the pedestal. Typically, an RF power supply is coupled to a pedestal electrode through a coupling capacitor, and a negative DC self-bias voltage develops on the pedestal adjacent to the plasma.
At least two techniques are available which increase the plasma density in the sputtering chamber and thereby increase the fraction of ionized sputtered atoms.
One method, called ionized metal plating (IMP), uses an RF inductive coil wrapped around the processing space between the target and the wafer to couple RF energy in the megahertz frequency range into the processing space. The coil generates an axial RF magnetic field in the plasma which in turn generates a circumferential electric field at the edges of the plasma, thereby coupling energy into the plasma in a region remote from the wafer and increasing its density and thereby increasing the metal ionization rate. IMP sputtering is typically performed at a relatively high argon pressure of 50 to 100 milliTorr.
IMP is very effective at deep hole filing. Its ionization fraction can be well above 50%. However, IM Pequipment is relatively expensive. Even more importantly,IMP tends to be a hot energetic, high pressure process in which a large number of argon ions are also accelerated toward the wafer. Film quality resulting from IMP is not optimal for all applications.
A recently developed technology of self-ionized plasma (SIP) sputtering allows plasma sputtering reactors to be only slightly modified but to nonetheless achieve efficient filling of metals into high aspect-ratio holes in a low-pressure, low-temperature process. This technology has been described by Fu et al. in U.S. patent application Ser. No. 09/546,798, filed Apr. 11, 2000, and by Chiang et al. in U.S. patent application Ser. No. 09/414,614, filed Oct. 8, 1999, both incorporated herein by reference in their entireties.
The SIP sputter reactor described in the above cited patents is modified from a conventional magnetron sputter reactor configured for single-wafer processing. SIP sputtering uses a variety of modifications to a fairly conventional capacitively coupled magnetron sputter reactor to generate a high-density plasma adjacent to the target and to extend the plasma and guide the metal ions toward the wafer. Relatively high amounts of DC power arc applied to the target, for example, 20 to 40 kW for a chamber designed for 200 mm wafers. Furthermore, the magnetron has a relatively small area so that the target power is concentrated in the smaller area of the magnetron, thus increasing the power density supplied to the HDP region adjacent the magnetron. The small-area magnetron is disposed to a side of a center of the target and is rotated about the center to provide more uniform sputtering and deposition.
In one type of SIP sputtering, the magnetron has unbalanced poles, usually a strong outer pole of one magnetic polarity surrounding a weaker inner pole. The magnetic field lines emanating from the stronger pole may be decomposed into not only a conventional horizontal magnetic field adjacent the target face but also a vertical magnetic field extending toward the wafer. The vertical field lines extend the plasma closer toward the wafer and also guide the metal ions toward the wafer. Furthermore, the vertical magnetic lines close to the chamber walls act to block the diffusion of electrons from the plasma to the grounded shields. The reduced electron loss is particularly effective at increasing the plasma density and extending the plasma across the processing space.
Gopalraja et al. disclose another type of SIP sputtering, called SIP+ sputtering, in U.S. patent application Ser. No. 09/518,180, filed Mar. 2, 2000, also incorporated herein by reference in its entirety. SIP+ sputtering relies upon a target having a shape with an annular vault facing the wafer. Magnets of opposed polarities disposed behind the facing sidewalls of the vault produce a high-density plasma in the vault. The magnets usually have a small circumferential extent along the vault sidewalls and are rotated about the target center to provide uniform sputtering. Although some of the designs use asymmetrically sized magnets, the magnetic field is mostly confined to the volume of the vault.
SIP sputtering may be accomplished without the use of RF inductive coils. The small HDP region is sufficient to ionize a substantial fraction of metal ions, estimated to be between 10 and 25%, which is sufficient to reach into deep holes. Particularly at the high ionization fraction, the ionized sputtered metal atoms are attracted back to the targets and sputter yet further metal atoms. As a result, the argon working pressure may be reduced without the plasma collapsing. Therefore, argon heating of the wafer is less of a problem, and there is reduced likelihood of the metal ions colliding with argon atoms, which would both reduce the ion density and randomize the metal ion sputtering pattern.
However, SIP sputtering could still be improved. The ionization fraction is only moderately high. The remaining 75 to 90% of the sputtered metal atoms are neutral and not subject to acceleration toward the biased wafer. This generally isotropic neutral flux does not easily enter high-aspect ratio holes. Furthermore, the neutral flux produces a non-uniform thickness between the center and the edge of wafer since the center is subjected to deposition from a larger area of the target than does the edge when accounting for the wider neutral flux pattern.
One method of decreasing the neutral flux relative to the ionized flux is to increase the throw of the sputter reactor, that is, the spacing between the target and pedestal. For a 200 mm wafer, a conventional throw may be 190 mm while a long throw may be 290 mm. Long throw may be defined as a throw that is greater than 125% of the wafer diameter. In long throw, the more isotropic neutral flux preferentially deposits on the shields while the anisotropic ionized flux is not substantially reduced. That is, the neutrals are filtered out.
However, long-throw sputtering has drawbacks when combined with SIP sputtering relying upon an unbalanced magnetron to project the magnetic field toward the wafer. The vertical magnetic component is relatively weak and rapidly attenuates away from the target since it necessarily returns to the magnetron. It is estimated that for a typical unbalanced magnetron producing a 1 kilogauss horizontal magnetic field at the target produces only a 10 gauss vertical magnetic field 100 mm from the target, and it rapidly decreases yet further away. Therefore, an unbalanced magnetron in a long-throw sputter reactor does not provide the magnetic plasma support and magnetic guidance close to the wafer that is needed to obtain the beneficial results of SIP sputtering.
Another problem arises in SIP sputtering using a strongly unbalanced magnetron because the vertical components of the magnetic field close to the wafer are invariably non-uniform as they are being attracted back toward the magnetron. Such non-uniformity in the magnetic field is bound to degrade the uniformity of sputtering across the wafer.
Also, in SIP+ sputtering with the vaulted target, there is relatively little magnetic field extending out of the vault to support the plasma and guide the metal ions toward the wafer.
Accordingly, it is desired to provide a better alternative for magnetic confinement and guidance of ionized sputtered atoms.
In a magnetron sputter reactor, a coil is wrapped around the processing space between the target and pedestal supporting the substrate being sputter coated. The coil is powered, preferably by a DC power supply, to generate an axial field in the sputter reactor. The axial magnetic field is preferably in the range of 15 to 100 gauss.
The magnetron preferably is unbalanced with a stronger pole surrounding a weaker inner pole of the opposed magnetic polarity. The stronger pole preferably generates a magnetic flux parallel to the magnetic flux generated by the coaxial DC coil.